Joints in the body are comprised of bones with articulating surfaces that move against or in close proximity with one another in the joint. For example, the knee is a joint that includes the distal end of the femur that terminates into the medial and lateral condyles, which cooperate with both the patella and the meniscus of the proximal tibia for joint movement. Bone disease, such as arthritis, may deteriorate one or more joint compartments so that movement in the joint produces pain. One remedy for joints deteriorated by disease or injury is replacement of one or more compartments of the joint with artificial implants. For example, a total knee replacement (TKR) may be performed by surgically opening the knee, removing the diseased portions of the distal femur, proximal tibia, and/or patellar surface, and implanting an artificial prosthesis. The biocompatible materials used to make the bone prosthesis reduce the risk of infection and wear well.
One important aspect of using artificial implants to repair a diseased joint is the fit of the artificial implants with one another and the patient's joint physiology. That is, implant size, shape, and location are important parameters for enabling patient movement that complies with the ligaments and muscles of the patient. During the recovery and rehabilitation period following a surgery, information regarding the placement and stability of the implant components is useful to determine whether the surgery went as planned and whether healing is occurring as expected.
Data regarding the implant components and their integration into the patient's joint may be obtained using radiographic images, magnetic resonance imaging, and computerized tomography scans. These methods produce images of the joint at particular points in time in the rehabilitation period, but they suffer from a number of shortcomings. For one, some of these techniques require the joint to be immobilized, otherwise image quality is compromised. For another, images typically cannot be generated of the joint while the patient is performing exercises or other rehabilitation activities. Additionally, images developed by these techniques are not real-time nor do they provide measurements of forces active in the joint. Continuous and accurate information regarding the implant components during rehabilitation sessions would have significant value in the evaluation of a joint replacement surgery.
In an effort to provide parameter measurements in the vicinity of orthopaedic components implanted in a patient, sensors and telemetry modules, which transmit the parameters measured by the sensors, have been coupled with bone fixation devices and bone fusing implants. For example, U.S. Pat. No. 6,034,296 discloses a bone fixation device to which a strain sensor and a telemetry unit are mounted. The strain sensor generates a signal that corresponds to the magnitude of the strain being sensed by a bone fixation device being used to repair a fracture. The signal is provided by a conductor to the telemetry unit so the signal may be used to modulate a signal transmitted over an antenna that is coupled to the telemetry unit. In U.S. Pat. No. 6,706,005, a sensor and a telemetry module are associated with a ventral cervical stabilization system to provide data regarding a bone fusion on a patient's spinal column. Again, data derived from one or more sensors coupled to the telemetry unit are wirelessly communicated to a device external to the patient by modulating a signal from the telemetry unit with the data and radiating the resulting signal through an antenna. Wireless communication is preferred because wires extending through the skin of the patient present opportunities for infections.
In these previously known wireless communication systems that may be implanted in a patient, the antenna is a component of the telemetry module that must be micro-machined or micro-fabricated for wireless communication with a communication unit that is external to the patient's body. The antenna may be incorporated in the telemetry module or sensor. Alternatively, the antenna may be manufactured as a separate component in the system and coupled to the telemetry unit. Consequently, the antenna design and manufacture further complicates the inclusion of the parameter sensor and electronics module with a bone fixation device or orthopaedic component.
What is needed is a way of providing continuous and real-time data from a joint in which a complete or partial joint replacement surgery has occurred without requiring antenna micro-machining or micro-fabrication in the telemetry unit.
What is needed is a way of providing continuous and real-time data from a joint in which a complete or partial joint replacement surgery has occurred without requiring the manufacture of a separate antenna component for the system.